INNOVATIVE BASE ISOLATORS FROM SCRAP TYRE RUBBER PADS · “Seismic design of RC buildings - Theory and Practice”, with Springer. The book received The book received award from

INNOVATIVE BASE ISOLATORS FROM

SCRAP TYRE RUBBER PADS

Suhasini N Madhekar1, Harshavardhan Vairagade2

1. Applied Mechanics Division, College of Engineering Pune, India

2. Design Innovation Center, College of Engineering Pune, India

ABSTRACT. Attenuating the effects of severe ground motions on buildings is always one

of the most popular topics in structural engineering research. The reduction of seismic

demand on structures can be achieved by providing certain degree of flexibility in the

structure by installing the devices having low horizontal stiffness. Among these, elastomeric

bearings, sliding bearings and hybrid systems are the most widely used. The introduction of

flexible layer increases the deflection of the structure, thereby increasing the time period of

the structure and decreasing the base shear. The Eco-friendly Scrap Tyre Rubber Pads

(STRPs) provide several advantages such as low-cost, ease of handling and, simple shear

stiffness adjustments, by changing the number of layers. They also provide environmental

benefits, by recycling scrap tyres unlike other commercially available base isolators. In the

present study, the properties of STRP specimen are evaluated experimentally. The STRPs are

prepared by inserting layers of thin steel shims between rubber pads. Steel plates are provided

at top and bottom and the entire assembly is subjected to vulcanization process. The tests

conducted are (a) axial compression test and (b) horizontal shear test. Using the properties of

STRPs obtained experimentally, a base-isolated G + 8 Reinforced Concrete (RC) building is

analysed on software ETABS. It is found that there is considerable reduction in the base shear

and storey drift by installation of STRPs. Hence the innovative base isolators from Scrap

Tyre Rubber Pads can be used for low rise structures as base isolators.

Keywords: Base isolation, Scrap Tyre Rubber Pads, Elastomeric Isolators, Laminated

Rubber Bearings

Dr Suhasini Madhekar is from Applied Mechanics Division at College of Engineering

Pune. Her research interests include Structural Dynamics, Vibration Control, Bridge

Engineering and protection of structures from earthquake. She has authored a book on

“Seismic design of RC buildings - Theory and Practice”, with Springer. The book received

award from Association of Consulting Civil Engineers, India, as ‘Best Publication of the year

2017- useful to structural consultants.

Harshavardhan Vairagade is Research Associate at Design Innovation Center, College of

Engineering Pune. His interests include development of low-cost products of practical utility.

INTRODUCTION

This paper focuses on the experimental and analytical studies conducted on the development

of low-cost seismic isolation pads, using scrap automobile tyres. Seismic isolation is a well-

defined building protection system against earthquakes. The majority of the previous studies

focus on the performance improvement of the structure, when installed with base isolation

systems. However, this study aims at cost and weight reduction of seismic isolation pads by

recycling; otherwise useless material - scrap tyres. Elastomer-based isolators have been

widely studied and used globally (Matsagar and Jangid) [3].

Steel or fiber reinforcement inside the elastomeric isolators provides high vertical stiffness,

whereas rubber segments between reinforcement layers provide low horizontal stiffness.

Automobile tyres have a similar effect as the steel plates or fibers inside the conventional

elastomer-based isolators. Therefore, rectangular shaped layers cut from tread sections of

used tyres and then piled on top of each other, can function as an elastomeric bearing. STRPs

may be used as a low-cost alternative to conventional elastomeric bearings for seismic

isolation of massive structures, masonry structures, pedestrian bridges, equipment isolation

and such other applications.

LITERATURE REVIEW

Mishra and Igarashi (2012) [4] conducted experiments as well as analytical study on layer-

bonded Scrap Tyre Rubber Pad (STRP) isolators to develop low-cost seismic isolators. The

vertical compression and horizontal shear tests were conducted with varying axial loads and

the test results were used to compute different mechanical properties of the STRP isolators,

including vertical stiffness, horizontal stiffness and damping ratios. The tested STRP samples

showed energy dissipation capacity considerably greater than the natural rubber bearings.

Turer and Ozden (2008) [5] conducted research to develop no-cost seismic base isolation

pads using scrap automobile tyre pads. The mechanical and dynamic properties of STRP

specimens made from different tyre brands, with different number of layers and orientations

were evaluated experimentally. The results of these STRP tests were compared among

themselves and against a commercially available Laminated Rubber Bearing (LRB)

specimen. Static and dynamic tests conducted on STRP samples showed similarities between

STRP and Lead Rubber Bearing (NZ - New Zealand) bearing and high-damping rubber

bearing response.

SEISMIC ISOLATION

The seismic forces on the structures can be reduced if the fundamental period of the structure

is lengthened or the energy dissipating capability is increased. Therefore, seismic isolation is

a promising alternative for earthquake-resistant design of structures. Seismic isolation is

essentially a method of controlling the seismic response of structures through yielding of the

isolators, possessing generally bilinear force deformation relationship. Conceptually, seismic

isolation decouples the structure from the horizontal components of the ground motion by

interposing structural elements with low horizontal stiffness between the structure and the

foundation. This gives the structure a fundamental frequency that is much lower than both; its

fixed-base frequency and the predominant frequencies of the earthquake ground motion

(Kelly, 1997) [2].

Elastomeric bearings consist of multiple layers of elastomer and steel shims, which carry

gravity load of the superstructure and provide horizontal flexibility to reduce the level of

seismic forces transmitted to the superstructure. The reason behind the popularity of the

elastomeric bearings is the restoring force provided by the elastomeric material.

SCRAP TYRE RUBBER PADS (STRP)

The aim of the research is to develop low-cost isolation device for structural applications. To

address this issue, the materials of the base isolation system should be easily available at an

affordable cost. The rubber and the steel reinforcing wires used in manufacturing tyres are the

alternative materials of the proposed isolation system. Usually, the Radial tyre contains steel

reinforcing cords in the form of layers oriented in specified directions with respect to the

Radial direction of tyre. The steel reinforcing wires are supposed to represent the steel plates

used in conventional steel laminated elastomeric bearings, preventing the lateral bulging of

the rubber bearings. In addition, these reinforcing cords are responsible to provide adequate

vertical stiffness to the isolator. The purpose of the study is to check the feasibility of using

the STRP as seismic isolator.

Objectives of Study

In the recent years, several practical techniques for achieving base isolation and a variety of

energy dissipating devices have been developed and implemented around the world. Due to

the size, weight and incurred cost of the available seismic isolation devices, the use of this

technology is almost out of reach in the developing countries. This research work proposes an

alternative seismic isolation system, particularly suitable for developing countries, making

use of used tyre rubber. The primary objective of the research is to develop a base isolation

system, which shall be effective in reducing the seismic demand on structures, made from

easily available materials, at an affordable cost. Experimental investigations on specimen

isolation bearings, in order to identify the behavior of the proposed seismic isolator with

vertical compressive and cyclic shear loadings have been performed. The test results are used

for numerical study of a five-storey Reinforced Concrete (RC) hospital building, to check the

viability of the STRP.

Types of Tyres

Two types of tyres are available, viz. Nylon tyres and Radial tyres.

Nylon Tyres:

Nylon Tyres, also known as Cross Ply tyres, consist of layers made from Nylon cord. Cross

Ply tyres provide a strong and rigid sidewall causing the tyre to overheat, when used on a

hard road surface and thus causes the tyre to wear out quickly. Thus, these types of tyres are

not suitable for trucks and buses.

Radial Tyres:

The Radial tyres consist of steel reinforcement vulcanized with rubber. The flexibility and the

strength help the Radial tyres to absorb the impact shocks and the bumps, more effectively

than the cross ply tyres. The vertical load carrying capacity of the isolators prepared from

Nylon tyres is less, as compared to the Radial tyres, as the Radial tyres consist of steel mesh,

which helps them to bear larger vertical loads.

Layer-Bonded STRP Specimen

Several samples of STRP and layer-bonded STRP bearings as shown in figure 1 were

fabricated, for the purpose of vertical compression and shear loading tests. The specimen

samples were prepared by using scrap bus / truck tyres. Square samples of size 150 mm and

200 mm were prepared. The number of strips to be used for the preparation of sample

generally varies from a minimum of four to maximum of eight. More the number of strips,

more flexible are the STRP bearings in horizontal direction; however, it results in the

reduction in the stability of the samples. In this study, samples were prepared using four

strips of tyres.

The layers were glued together for better bonding and to keep them intact and act like a

single unit. Epoxy RB -106 was used as an adhesive. Adhesive (resin +hardener) was

uniformly applied to each layer. In preparing the samples, thickness of each layer was 15 mm

and thickness of applied epoxy layer was 2 mm. Using four such layers, the total thickness of

the STRP was 66 mm. Two pieces of plywood, one above the sample and the other below the

sample were placed to fix the C-clamp, to provide load on the sample for their proper

bonding. The samples were then kept undisturbed for 7 days, as the epoxy requires 7 days for

curing.

Figure 1 Stages in Preparation of the STRP Samples

(a)

(d)

(b)

(e)

(c)

EXPERIMENTAL STUDY ON THE LAYER-BONDED STRP

SPECIMEN

Testing Machine

To investigate the properties of the layer bonded STRP samples, axial compression test and

horizontal shear test were performed. The maximum vertical and horizontal load carrying

capacity of the machine was 15,000 kN and 10,000 kN respectively. The standard stress to

equivalent load conversion was 1 N/mm2. Figure 2 shows the test setup.

Figure 2 Setup for Axial Compression Test

Axial Compression Test

In order to investigate the vertical load carrying capacity and vertical stiffness of the layer-

bonded STRP, axial compression test was performed. Six samples of Nylon tyre and three

samples of Radial tyre were tested. One sample from both types of tyres was tested to

determine the maximum load carrying capacity until failure, initiated by severe cracking.

Remaining samples were tested for cyclic vertical axial compression loading.

Testing and Analysis of Nylon Tyre Sample for Axial Compression

The first STRP Nylon tyre sample was tested for pure axial compression, in order to find the

vertical load bearing capacity. It was placed between two steel plates, one above and the

other at the bottom, for uniform application of load. Two dial gauges were placed in order to

measure the vertical deformation of the sample. The deflection was measured at an equal load

interval of 0.2 N/mm2 until the sample failed. The failing of sample was noticed, as the

sample was not able to carry any further load. The load factor applied as per the testing

equipment was 1N/mm2 = 363 kN. Maximum vertical load carrying capacity obtained was

363 kN. The load-deflection curve is as shown in figure 3.

Figure 3 Deflection Against Vertical Compressive Load for Pure Axial Loading of Nylon

Tyre Sample

TESTING AND ANALYSIS OF NYLON TYRE SAMPLE FOR

VERTICAL STIFFNESS

Cyclic loading test was carried out on a Nylon tyre sample to find the vertical stiffness. The

experimental setup was same as that for axial compression test. Two cycles of loading were

applied on this sample. In the first cycle, the load was increased at equal intervals of

0.1N/mm2, 0.2N/mm2, 0.3N/mm2 and 0.35N/mm2. After applying the load of 0.35N/mm2, the

load was made zero. In the second cycle, the load was increased at equal intervals of 0.1

N/mm2 up to 0.7 N/mm2 and then it was further reduced to zero. The corresponding

deformations were noted for both cycles. The cyclic variation of defection with vertical load,

for a typical sample case is shown in figure 4.

Figure 4 Cyclic Variation of Deflection for Nylon Tyre Sample

Equation of curve is

At (average deflection), MN/m

Therefore, vertical stiffness of the Nylon tyre sample is 34.16 MN/m

TESTING AND ANALYSIS OF RADIAL TYRE SAMPLE FOR

VERTICAL STIFFNESS

Maximum vertical load carrying capacity of Radial Tyres was obtained was 515.5 kN Cyclic

loading test was carried out on this sample to find the vertical stiffness. In the first cycle load

was increased at intervals of 0.1 N/mm2 up to 0.35 N/mm2 and the deflections corresponding

to this incremental loading were noted down with the help of two dial gauges. After reaching

to a load of 0.35 N/mm2, load was removed. In the second cycle, load was increased at equal

intervals of 0.1N/mm2 up to 0.7N/mm2 and then again reduced it to zero and; the

corresponding deformations were noted. A typical load deflection curve for vertical

deflection of Radial tyre STRP is shown in figure 5.

Figure 5 Cyclic Variation of Vertical Deflection for Radial Tyre Sample

Equation of curve is

………… (Kv indicates vertical stiffness)

At (average)

MN/m

Therefore, the vertical stiffness of the sample is 46.35 MN/m. Following the same procedure,

two more samples were tested. The test results for Nylon and Radial tyres, tested in vertical

cyclic loads are presented in Table 1. It presents vertical stiffness for Nylon and Radial tyre

samples. The average vertical stiffness of Radial tyre STRP is found to be 1.45 times that of

the Nylon tyre samples.

Table 1 Vertical Cyclic Load Test

HORIZONTAL SHEAR TEST

In order to investigate the horizontal load bearing capacity, horizontal stiffness and the

damping ratio of layer-bonded STRP, horizontal shear tests on both type of samples i.e.

Nylon tyre and Radial tyre were conducted. The testing was done on three pairs of samples

for both the type of tyres. Out of three specimens, first sample was tested only to determine

horizontal load bearing capacity. Other pairs of sample were tested to determine horizontal

stiffness. Setup for horizontal testing is shown in figure 6.

Sr. No. Tyre sample Type Average vertical deflection

(mm)

Vertical Stiffness (MN/m)

1 Nylon 1.37 34.16

2 Nylon 1.63 36.37

3 Nylon 1.72 27.16

4 Nylon 3.57 24.10

Average vertical stiffness of Nylon Tyres 30.5 MN/m

5 Radial 2.19 46.35

6 Radial 6.86 42.00

Average vertical stiffness of Radial Tyres 44.175 MN/m

Horizontal Shear Test on Nylon Tyre Sample Pair 1:

The first sample was tested in order to find its horizontal load bearing capacity. Two samples

were placed between three steel plates as shown in the figure 6. A constant vertical force of

450kN was applied. The samples were then horizontally sheared by applying the horizontal

load on the middle plate and the corresponding lateral displacements of the sample were

noted. The horizontal load bearing capacity of the sample was found to be 1110.8 kN.

Horizontal Shear Test on Nylon Tyre Sample Pair2:

In the first cycle, the load was increased at equal intervals of 0.2N/mm2 up to 0.8 N/mm2 and

the corresponding deformations were noted. After applying the load of 0.8N/mm2, the load

was made zero. In the second cycle, the load was increased at equal intervals of 0.4N/mm2

up to 1.6N/mm2 and then again reduced to zero. The corresponding deformations were noted.

Figure 7 shows variation of deflection with horizontal load for Nylon tyre sample.

Figure 6 Setup for Horizontal Shear Test on STRP

Kh = = = 89.64 kN/m

where, f (+), f(-), δ(+), δ(-) are peak values of horizontal force and deflections respectively.

Therefore the horizontal stiffness for this sample is 89.64 kN/m

Figure 7 Deflection Against Horizontal Force for Cyclic Loading of Sample Pair 2 of Nylon

Tyre

Horizontal Shear Test on Radial Tyre Sample Pair 1:

The first sample was tested in order to find its horizontal load bearing capacity. Two samples

were placed between three steel plates as shown in the figure 6. A constant vertical force of

508 kN was applied. The samples were then horizontally sheared by applying the horizontal

load on the middle plate and the corresponding lateral displacements of the sample were

noted. The horizontal load bearing capacity of the sample was found to be 1216.04 kN.

Horizontal Shear Test on Radial Tyre Sample Pair 2:

In the first cycle, the load was increased at equal intervals of 0.3N/mm2 up to 1.2 N/mm2 and

the corresponding deformations were noted. After applying the load of 1.2N/mm2, the load

was made zero. In the second cycle, the load was increased at equal intervals of 0.4N/mm2

up to 1.6N/mm2 and then again reduced to zero. The corresponding deformations were noted.

The horizontal load deflection curve obtained is shown in figure 8.

Kh = = = 126.15 kN/m

where, f(+), f(-), δ(+), δ(-) are peak values of horizontal force and deflections respectively.

Therefore, the horizontal stiffness for this sample is 126.15 kN/m.

Figure 8 Deflection against Horizontal Force for Cyclic Loading of

Radial Tyre Sample Pair

Table 2 Cyclic Loading Horizontal Shear Test

Sr. No. Tyre sample Type Horizontal Stiffness (Kh) (kN/m) % Damping

1 Nylon 89.64 7.20

2 Nylon 96.04 7.63

3 Nylon 92.56 7.03

4 Nylon 98.16 7.91

Average horizontal stiffness of Nylon Tyres = 94.10 kN/m

Average % Damping = 7.44 %

Average damping for Nylon Tyres = 7.44%

5 Radial 126.15 17.7

6 Radial 132.65 19.26

Average horizontal stiffness of Radial Tyres = 129.4 kN/m

Average % Damping = 18.48 %

Average damping for Radial Tyres = 18.48 %

Table 2 presents horizontal stiffness and damping values for Nylon and Radial tyre samples.

Table 3 presents the ratio of vertical to horizontal stiffness for both types of tyres.

Table 3 Ratio of Vertical to Horizontal Stiffness of Tyres

Sr. No. Tyre Sample Kv (Average)

(kN/m)

(kN/m)

Kh (Average)

(kN/m)

Ratio (Kv/ Kh)

1 Nylon 30,500 94.10 324.123

2 Radial 44,175 129.40 341.383

DAMPING RATIO ()

The damping ratio was evaluated from the area under the curve, for each sample.

β =

where, δ(+), δ(-) are peak values of horizontal deflections.

Nylon Tyre Sample

The equation of curve for Nylon tyre sample from figure 7 is considered. The area under the

curve is evaluated by integrating the equation of the curve between 0 to the maximum value

of deflection. The horizontal stiffness is taken for sample 1, presented in table 2. Typical

calculations for evaluating damping for both types of samples are presented below.

Equation of curve = -2.612x2+ 67.01x+ 155.3

Eloop = = 4603.545

β = = 7.2%

Radial Tyre Sample

β =

Equation of curve = -48.26x2+ 297x+ 160.1

Eloop = = 1988.053

β = = 17.7%

ANALYSIS ON ETABS

An eight - storey hospital building located in seismic zone III was analysed on software

ETABS, considering Importance factor 1.5 and Soil bearing capacity 300 kN/m2. The plan

dimensions of the building were 54.931m × 38.404 m and the height of the building was 29.5

m, with floor-to-floor height of 3.5m. Figure 9 shows 3D model of the hospital building in

ETABS.

Figure 9 3 D Model of Hospital Building

The building was analyzed for two cases, (i) fixed base case and (ii) isolated base case. In the

second case, properties of Radial tyre STRPs, evaluated experimentally, were used.

Table 4 Time Period and Spectral Acceleration of G + 8 RC Building

Mode Time Period (sec)

Without Isolator (Sa/g)1

Time Period (sec)

With Isolator (Sa/g)2 % Difference

1 1.285 1.05 1.608 0.5295 49.57%

2 1.162 1.170 1.467 0.584 50.08%

3 1.115 1.219 1.377 0.6177 49.32%

4 0.424 2.50 0.471 1.576 36.96%

5 0.413 2.50 0.433 1.576 36.96%

6 0.375 2.50 0.426 1.576 36.96%

7 0.360 2.50 0.408 1.576 36.96%

8 0.300 2.50 0.322 1.576 36.96%

Using response spectrum method of analysis, the spectral acceleration for each mode was

evaluated. Table 4 presents the time period and spectral acceleration values for first 8 modes

of the building.

CONCLUSIONS

The time period for the building in all modes is found to increase after the installation of the

Scrap Tyre Rubber Pads. This in turn decreased the value of Sa/g and thus reduced the value

of Base Shear.

Maximum percentage difference for the spectral acceleration coefficient (Sa/g) between the

time period without isolator and the time period with isolator is found to be 50.08 %. Average

percentage difference for the spectral acceleration coefficient (Sa/g) between the time period

without isolator and the time period with isolator is found to be 41.72%. As base shear is

directly proportional to the spectral acceleration coefficient (Sa/g) as per IS 1893 part I

(2002) clause 6.4.2, the value of base shear will reduce proportionally. Therefore, the

installation of STRP isolators in the building has increased the time period and decreased the

base shear to 50%, functioning like a conventional isolator. The minimum required ratio

between vertical and horizontal stiffness is 150. Hence, the layer-bonded STRP bearings can

be used for seismic isolation purpose

1. During the shear deformation, samples did not show layer separation. This

indicates that the bonding agent is sufficiently strong in transmitting the shear forces.

2. The damping ratio of the tested STRP isolator made of Radial tyres (18.48%) is

found to be greater than the damping ratio of the natural rubber bearings (3.5%).

Therefore, the use of additional damping enhancement mechanisms can be avoided

when using the STRP isolator for a specified level of damping.

3. Base shear force transmitted to the superstructure is reduced to 50% of that for

the fixed base building. These results show that the base isolation system using STRP

isolators is an attractive alternative to commercially available isolation systems.

4. The cost of one sample of Radial tyre (of dimensions 200 mm × 200 mm × 80

mm) works out to be Rs. 630/-, which is easily affordable, compared to the cost of

conventional isolators.

5. The objective of making an isolator, which is easily reproducible, is also

achieved, as the STRP requires no heavy machinery and no initial hefty investment.

REFERENCES

1. IS 1893 Part 1, Criteria for earthquake resistant design of structures, General

provisions and Buildings, 2002

2. Kelly J., Earthquake resistant design with rubber, Springer, London, 1997

3. Matsagar V., Jangid R., Influence of isolator characteristics on the response of base-

isolated structures, Engineering Structures, Vol. 26, 1735-1749, 2004

4. Mishra H., Igarashi A., Experimental and analytical study of scrap tyre rubber pad for

seismic isolation, International Journal of Civil and Environmental Engineering 6,

2012

5. Turer A., Bayezid Z., Seismic base isolation using low-cost Scrap Tyre Pads (STRP),

Materials and Structures 41:891–908, 2008